Individual study: Springtime drawdowns to restore coastal wetlands, restricts tidal exchange and diminishes the nursery function of coastal marshes in the Barataria Basin, Louisiana, USA
Rozas L. P. & Minello T. J. (1999) Effects of structural marsh management on fishery species and other nekton before and during a spring drawdown. Wetlands Ecology and Management, 7, 121-139
The loss of coastal wetlands and fishery habitat along the northern Gulf of Mexico is a severe problem. Structural marsh management, which incorporates periodic drawdowns of the managed area, has been advocated as one method to restore coastal wetlands, especially those marshes thought to be degraded by saltwater intrusion. During a drawdown, the water level is lowered by allowing water out, but not into the managed area. Marsh management projects often have been implemented without adequate monitoring to determine the consequences to fisheries. The objective of our study was to test the effects of structural marsh management on habitat use by juvenile fishery species and other small nekton before and after a spring drawdown of managed areas.
Study site: The study was conducted in the Barataria Basin of coastal Louisiana approximately 35 km south of New Orleans (29º 35' N, 90º 5' W), southeast USA. An impoundment with water-control structures (managed area) was constructed and an adjacent reference (unmanaged area) established at each of two locations within the study site in cooperation with local landowners to assess the effects of structural marsh management in the Mississippi River deltaic plain. These four experimental sites contained both emergent marsh and open water (ponds) at the time of study. Reed et al. (1997) provide a detailed description of the management regime used to control water levels in the two managed areas (impoundments). Water level was manipulated in each managed area using one 1.05-m diameter, flap-gated culvert with installed stop-logs.
Methods: Samples of nekton were collected during two periods (March 21-24 and May 16-19) in the spring of 1995, targeting the time when a drawdown was initiated (March 16) and two months into the drawdown. These sampling times also coincided with an important period of recruitment for marine fishery species along the northern Gulf coast when nekton abundance in the study area is expected to be high. During each period, a variety of 1m² enclosure samplers were used determined mainly by water levels, to collect samples in the managed and reference sites. Densities of abundant species identified from these samples were compared between managed and unmanaged areas. Standing crop (number or biomass of nekton per hectare of marsh area) between areas was also compared. Standing crop was estimated by combining habitat-specific densities with the area of different habitat types within each managed or unmanaged area.
In March, densities of resident taxa, which complete their life cycles within the estuary, were significantly greater in the managed areas compared to the unmanaged areas. The densities of most resident species were either similar between areas, or significantly greater in unmanaged areas during the drawdown (May). In contrast to resident species, the transient fishery species (e.g., blue crab Callinectes sapidus and brown shrimp Farfantepenaeus aztecus) reproduce outside the marsh system and recruit to these areas as young. The densities of these transient species were significantly higher in unmanaged areas during both sampling events. Standing crops of transient species also were substantially greater in unmanaged than managed areas.
Conclusions: Our study showed that restricting the water exchange to tidal marshes under structural marsh management diminishes the recruitment and standing stocks of fishery species that must migrate from nearshore or offshore spawning sites to marsh nursery areas. The practice of drawing down a management area essentially eliminates recruitment of fishery species into the area and may result in a complete loss of the area’s nursery function during the time the drawdown is in effect. Even when water-control structures are open, the densities of fishery species are reduced inside the managed areas because water exchange is reduced compared to unmanaged marshes (Rogers et al. 1994). In contrast to the negative effect of management on fishery species, resident fish and crustacean populations appear to flourish in managed areas when a drawdown is not in effect. During a drawdown, resident species populations decline as the area of submerged aquatic vegetation within the managed area also decreases.
Reed D. J., De Luca N. & Foote. A.L. (1997) Effect of hydrologic management on marsh surface sediment deposition in coastal Louisiana. Estuaries, 20, 301-311.
Rogers D.R., Rogers B.D. & Herke W.H. (1994) Structural marsh management effects on coastal fishes and crustaceans. Environmental Management, 18, 351-369.
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